Weight cargo is usually loaded onto ships by first loading it onto a large pallet, the whole pallet being transported by a transport car to the ship. The car is loaded onto the ship from an opening in the gunwale, and the cargo is then let down into the ship together with the pallet.
To unload cargo from the ship, time car boards the ship in the same way as for loading, receives the cargo together with the pallet, and is then transported onto land.
For this purpose a ramp is provided between the opening in the ship's gunwale and the wharf. As heavy transport cars generally have little capacity to climb hills, the slope of this ramp has to be gentle.
However, the slope of the ramp varies according to various conditions such as the height of the wharf, tide level, draft of the ship and clearance between the wharf and the ship's hull. The tide level changes with time and the draft of the ship changes with the progress of loading/unloading works, hence it is very difficult to maintain the ramp at a constant slope angle so that the transport car can always travel smoothly between the ship and the wharf.
In this context, Tokkai Hei 3-159897 published by the Japanese Patent Office discloses a loading and unloading device wherein a lift table is provided which can rise and fall to permit the transport car to board the ship, and to which one end of the ramp is attached by a hinge. In this arrangement, the slope of the ramp can be adjusted by raising or lowering the lift table.
However, changes in the slope of the ramp are due not only to the conditions above-described but to those which vary more dynamically. For example if the transport car is very heavy, the hull will tend to lean due to the displacement of the car on the ramp, and this too causes a change in the slope of the ramp. As a result, the bent portion of the ramp may scrape the underside of the car or the roof of the car may touch the upper part of the opening in the ship thus obstructing the car's passage, or the underside of the ramp may collide with the wharf and damage the ramp.
Using the aforesaid device, however, it was difficult to adjust the slope of the ramp with the right timing so as to compensate the inclination of the ship's hull due to the displacement of the transport car.